Chrome Frame is a browser plugin for Internet Explorer on Windows. It is designed to show certain websites written for it in the Chrome/Webkit browser engine instead of Internet Explorer's so they can use bleeding edge web standards and not worry about older Internet Explorer versions.
It can optionally be used to fully replace Internet Explorer's engine while still allowing one to use the Internet Explorer program.
Google has recently discontinued "support" for their Chrome Frame Internet Explorer plugin and as a result almost all downloads have disappeared from their website.
These are various older and newer versions of Chrome Frame I was able to find still hanging around public FTPs.